108 SAPROPHYTISM, PARASITISM AND PATHOGENISM 
majority of specific instances such a differentiation can be readily 
established. There is no hard and fast line of demarcation, how- 
ever, between organisms of the "opportunist" type and those pro- 
gressively pathogenic, for it is undoubtedly true that some "oppor- 
tunists" may exliibit epidemic tendencies for limited periods if a 
combination of conditions arise which favors the distribution of the 
organisms and either increases the invasive powers of the microbe or 
decreases the resistance of the host. The limited spread of such 
bacteria is far more frequently attributable to unusually direct transfer 
of organisms by a common vehicle to a series of susceptible hosts 
than to the escape of the microbes from one host to another. Thus, 
milk-borne epidemics of septic sore throat may be extensive and 
involve many patients, but secondary transfer from man to man is 
relatively uncommon. These bacteria have not, as a rule, perfected 
their mechanism of escape from the tissues of one host to those of 
another. The epidemics are usually of brief duration and it is prob- 
able that the surviving microbes return to their original parasitic 
state.^ 
Of far greater importance is a probable tendency of many progres- 
sively pathogenic bacteria to act more and more on the defensive; 
to gradually disembarrass themselves, on the one hand, of the offen- 
sive weapons which originally conferred upon their possessors the 
ability to invade their host, and, on the other hand, to perfect what- 
ever defensive weapons they may have possessed the rudiments of. 
Such a change, as Theobald Smith has pointed out, would be difficult 
to detect, because an elimination of the more aggressive type and its 
gradual replacement by a strain in which the defensive elements were 
more prominently represented would require years for its accomplish- 
ment. Such a change in the activities of the microorganisms would 
probably be accompanied by reciprocal activities of the host, so that 
eventually a strain of microorganisms would be evolved which has 
reached a state of relative equilibrium with the host. Unusually 
virulent strains of microbes would tend to perish with their hosts, 
and unusually susceptible hosts would tend to perish with their 
invaders. A mutual adjustment of virulence and resistance between 
the surviving hosts and microbes would lead eventually to one of 
three conditions.- 
1. Gradual extinction of the microorganism. 
2. The gradual assumption of a parasitic or "opportunist" exist- 
ence. 
3. A more perfect pathogenism in which the mechanism of invasion, 
multiplication within the tissues and escape to other hosts is accom- 
plished without acute damage to the host. 
1 See "Studies on Pathogenic B. coli from Bovine Sources" by Theobald Smith, 
Jour. Exper. Med., 1927, 46, 123-166, for important discussion. 
2 Theobald Smith (Some Problems in the Life History of Pathogenic Microorganisms, 
Am. Med., 1904, 8, 711) clearly stated and discussed this hiTJOthesis over two decades 
ago, and it is surprising how little cognizance has been taken of it. 
